High-speed fabrication of highly uniform ultra-small metallic microspheres

ABSTRACT

In a high-speed fabrication process for producing highly uniform ultra-small metallic micro-spheres, a molten metal is passed through a small orifice, producing a stream of molten metal therefrom. A series of molten metal droplets forms from the break up of the capillary stream. Applied harmonic disturbances are used to control and generate satellite and parent droplets. Significantly, the satellite droplets formed are smaller than the orifice, allowing for the production of smaller metal balls with larger orifices. The satellite droplets are separated from the parent droplets by electrostatic charging and deflection or by aerodynamic or acoustic sorting. Preferably, the satellite droplets are cooled before being collected to avoid defects and achieve high uniformity of the resulting metal balls.

[0001] This application is a divisional of U.S. application Ser. No.09/860,803 filed May 18, 2001, the contents of which are hereby fullyincorporated by reference. This application is related to provisionalU.S. application serial No. 60/206,507, filed May 22, 2000, the contentsof which are hereby fully incorporated by reference.

[0002] This invention was made with Government support under Grant No.DMI-9457205, awarded by NSF. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

[0003] The invention relates to methods of fabricating highly uniform,ultra-small metallic micro-spheres or balls, and to the balls themselvesfrom capillary stream break-up at high rates.

BACKGROUND OF THE INVENTION

[0004] The generation of droplets from capillary stream break-up hasbeen studied at least as early as Lord Rayleigh in the 1800s. Morerecently, the formation of metallic micro-spheres, or balls, from thebreak-up of a molten metal capillary stream has been studied. Such ballsare commonly used in the electronics industry for various applications,including interconnects for small electronics packages and in themanufacture of conductive pastes. Using the process of capillary streambreak-up, the balls can be produced at very high rates—typically tens ofthousands of droplets per second. Further, the nature of dropletformation due to capillary stream break-up results in highly uniformballs. The highly uniform size of the metal balls formed from capillarystream break-up is a significant improvement over other methods offorming conductive powders—such as spray atomization or meltspinning—which require the extra step of sieving or sorting thedifferently sized balls. This extra step is labor intensive,significantly increasing the time and cost of the manufacturing process;however, with such technologies, sorting or sieving is necessary toachieve tight ball diameter tolerances (on the order of five percent).

[0005] In the production of metal balls from capillary stream break-up,it is advantageous to effectively cool the balls so that they solidifybefore landing or bonding with each other. Effective solidificationreduces or eliminates (1) irregularly shaped balls that have dented whenthey impinge and (2) irregularly sized balls that have bonded togetherbecause they were insufficiently cooled. Without effectivesolidification, removal of these defects requires that the balls besieved or sorted.

[0006] Conventional methods of formation of metal balls due to capillarystream break-up tend to be limited to metal balls having diameters inexcess of 50 microns. A significant limitation on the size of metalballs produced from capillary stream break-up is the size of the orificefrom which the capillary stream emerges. Typically, droplets generatedfrom capillary stream break-up have diameters that are roughly twice aslarge as the diameter of the capillary stream orifice. The production ofsmaller balls, therefore, typically requires smaller orifices. As theorifice becomes very small, it tends to be more easily clogged by, e.g.,impurities in the molten metal. Further, obtaining smaller orifices thatare also uniform tends to be difficult and expensive. Currentstate-of-the-art provides a lower limit of orifice diameter availableoff-the-shelf and suitable for use with molten metals of 25 microns.

SUMMARY OF THE INVENTION

[0007] Accordingly, the present invention enables the formation ofmetallic micro-spheres due to capillary stream break-up that aresignificantly smaller than metallic micro-spheres formed by conventionalmethods and, more particularly, to metallic micro-spheres that aresignificantly smaller than the capillary stream orifice from which theyemerge, thereby overcoming many of the difficulties that plagued theprior art by advantageously enabling the formation of much smallermicro-spheres from larger orifices. The present invention furtherenables forming highly uniform metalic micro-spheres or balls, havingdiameters on the order of about 1 to 100 microns, and preferably lessthan 25 microns, without the defects and difficulties associated withconventional methods.

[0008] A method of manufacturing ultra-small metallic spheres comprisesdirecting a capillary stream of molten metal from an orifice by applyingan excitation disturbance, wherein the excitation disturbance isdetermined so that parent droplets and satellite droplets form from thestream due to capillary stream break-up. In one innovative aspect of thepresent invention, the satellite droplets are separated from the parentdroplets; cooled to form solid balls of substantially spherical shape;and collected as separate solid satellite balls. In another innovativeaspect of the present invention, the satellite and parent drops aresimultaneously cooled and collected as solid balls.

[0009] In one embodiment, the separating step is accomplished byelectrostatically charging the droplets and directing them through anelectric field, whereby the satellite and parent droplets deflectdifferently due to the different charge-to-mass ratios. In anotherembodiment, the droplets may be directed through a second electricfield, a rotating field, or both to further disperse the droplets. Ineither of these embodiments, the electrostatic charge may vary over timewhile the electric field remains constant or the electric field may varyover time while the electrostatic charge remains constant.

[0010] In accordance with another embodiment, separation of thesatellite and parent droplets is accomplished by acoustic forcing. Inaccordance with yet another embodiment, the satellite and parentdroplets are separated with aerodynamic forces.

[0011] In another innovative aspect, a solid metal ball of the presentinvention has a diameter that is preferably substantially less than thediameter of the capillary orifice. In a further innovative aspect, asolid metal ball of the present invention is substantially spherical andhas a diameter in a range of about 1.0 to 100 microns, and preferablyless than 25 microns. In yet a further innovative aspect, a metallicpowder comprises a plurality of such balls, wherein the balls are highlyuniform having a ball diameter tolerance of a mean ball diameter in therange of about 0.5 to 3.0 percent, and preferably less than 2.0 percent,without performing a mechanical sieving or sorting step.

[0012] In another innovative aspect of the present invention, the metalballs, satellite or both satellite and parent, are produced at a rapidrate, wherein the balls are highly uniform, having highly uniformdiameters. More particularly, the balls may be produced at a ratepreferably in a range of about 1000 to 200,000 balls per second, andpreferably at a rate greater than about 4000 balls per second whilemaintaining a ball diameter tolerance in the range of about 0.5 to 3.0percent, and preferably a ball diameter tolerance of less than about 2.0percent, without performing a mechanical sieving or sorting step.

[0013] Other aspects and features of the present invention will becomeapparent from consideration of the following description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross sectional view of the droplet generation system.

[0015]FIG. 2 is a side view of the capillary stream and satellitedroplet formation.

[0016]FIG. 3 is a schematic view of an embodiment for generatingsatellite droplets.

[0017]FIG. 4 is a graph of measured and theoretical droplet charge permass versus charge electrode voltage.

[0018]FIG. 5 is a graph of measured and theoretical droplet deflectiongiven deflection plate biasing.

[0019]FIGS. 6a and 6 b are is a schematic views of another embodimentfor generating satellite droplets.

[0020]FIG. 7 is a schematic view of another embodiment for generatingsatellite droplets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In accordance with the present invention, ultra small metal ballsor micro-spheres are produced at a high rate by capillary streambreak-up, wherein highly uniform and predictable droplets break from acapillary stream of molten metal. The present invention enables theformation of metallic micro-spheres due to capillary stream break-upthat are significantly smaller than metallic micro-spheres formed byconventional methods and, more particularly, to metallic micro-spheresthat are significantly smaller than the capillary stream orifice fromwhich they emerge, thereby overcoming many of the difficulties thatplagued the prior art by advantageously enabling the formation of muchsmaller micro-spheres from larger orifices. The balls may be formed fromone or a combination of various metals, including solder, copper,nickel, titanium, or any metal having physical properties (e.g., meltingpoint) suitable for the process described herein.

[0022]FIG. 1 shows a system 10 for producing metal balls in accordancewith one embodiment. To form a capillary stream, a droplet generator 12is provided. One example of a droplet generator suitable for thesepurposes is described in U.S. Pat. No. 6,186,192 to Orme et al., herebyincorporated in full by reference. This patent describes a system forgenerating a capillary stream of molten metal, from which a continuousseries of molten droplets form. Although the patent is directed toproducing droplets of molten solder, any metal having a suitable meltingpoint may be used therewith.

[0023] The droplet generator 12 includes a chamber 14 adapted to hold areservoir of molten metal 16 therein. As explained, this molten metalcomprises any metal having physical characteristics compatible with thesystem 10 and method described. The melting point of some metals, forexample, may be too high to use with the system 10 shown in FIG. 1. Avibrating rod 18 is slidably disposed within the chamber 14, contactingthe molten metal 16. The rod 18 is mechanically coupled to apiezoelectric crystal or transducer 20 and, as described, is used toimpart a disturbance in the molten metal. However, it should beappreciated that a disturbance may be imparted mechanically with apiezoelectric transducer with or without a rod or plunger—for example,the piezoelectric transducer may be placed under the orifice toeliminate the rod or plunger—or a disturbance may be imparted frommagnetic, electric or acoustic forces.

[0024] As shown, the piezoelectric crystal 20 is disposed outside thechamber 14 to protect it from the heat of the molten metal 16, aspiezoelectric materials can be damaged if subjected to hightemperatures. However, for metals with low melting points, such assolder, it may be possible to immerse the piezoelectric crystal in themolten fluid or position the piezoelectric crystal under the orificewhere temperatures are high. To further protect the piezoelectriccrystal 20 from heat transferred from the vibrating rod 18, a coolingjacket 22 may be attached to the vibrating rod 18, or to a housingaround the rod 18, near the crystal 20 to keep it at a coolertemperature. The cooling jacket 22 may be, for example, fluidly coupledto a circulating water supply that circulates room temperature waterthrough the cooling jacket 22. Additionally, to maintain the moltenmetal 16 inside the chamber 14 above its melting point, heaters 24 maybe coupled to the outer wall of the chamber 14 at spaced-apartlocations.

[0025] A controller 26, which may comprise one or more microprocessorsand one or more power supplies, is electrically coupled to thepiezoelectric crystal 20 by electrical connection 28. The controller 26delivers an alternating electrical signal to the piezoelectric crystal20, causing a corresponding mechanic response. The vibratingpiezoelectric crystal 20 causes the vibrating rod 18, to which thecrystal 20 is coupled, to oscillate. The vibrating rod 18 is preferablybiased with a periodic waveform, typically with a magnitude of about 50to 300 Volts, and a fundamental frequency ƒ, which corresponds to thefrequency of perturbation applied to the capillary stream for uniformdroplet production, determined by the following equation:$\begin{matrix}{{f = \frac{k \cdot V}{2\quad \pi \quad r_{o}}},} & (1.0)\end{matrix}$

[0026] wherein V is the droplet or stream velocity, r_(o) is the orificeradius, and k is a non-dimensional wavenumber constant, which depends onthe fluid properties of viscosity, surface tension and density, andambient gas density. See M. Orme, “On the Genesis of Droplet StreamMicrospeed Dispersions,” Physics of Fluids, 3, (12) pp 2936-2947, 1991.The constant k tends to vary between 0.4 and 0.8; for inviscid fluids, kequals 0.697. It should be appreciated that uniform droplets may beproduced at high rates and that the fundamental frequency ƒ variesaccording to orifice size and stream velocity. Preferably, thefundamental frequency ƒ, and thus the droplet production rate, is in arange of about 1000 Hz to 200 kHz.

[0027] The molten metal 16 is ejected from the chamber 14 through anorifice 30 in the bottom of the chamber 14, from which a stream 32 ofthe molten metal forms. The oscillation of the vibrating rod 18 producesa standing wave in the molten metal 16 and in the stream 32 as it leavesthe orifice 32. Due to capillary stream break-up, molten metal droplets34 form by detaching from the stream 32. A droplet 34 formed fromcapillary stream break-up has a diameter typically about twice thediameter of the orifice 30. With the current state-of-the-art ofoff-the-shelf orifices having diameters limited to 25 microns orgreater, the droplets formed from streams emerging from such orificestend to be in excess of 50 microns. To control the formation of moltenmetal droplets 34 leaving the droplet generator 12, a supply 36 deliversnitrogen gas (or other inert gas, such as argon) along a gas line 38 topressurize the chamber 14, thereby affecting the tendency of moltenmetal 16 to leave the chamber 14 through the orifice 30. Nitrogen (orother inert gas, such as argon) may also be supplied through a gas lineto a detachable end assembly to further control solder droplets.Preferably, the inert gas is a high purity gas, such as research gradeor better.

[0028]FIG. 2 illustrates the process of generating droplets fromcapillary stream break-up. An axisymmetric excitation disturbance isimparted to the stream 32 whose fundamental wavelength is in the regionof Rayleigh growth. As described above, the disturbance is imparted, inthis embodiment, by driving the piezoelectric crystal 20, to which thevibrating rod 18 is mechanically coupled, with an electrical signalrepresenting the disturbance via line 28. Alternatively, as describedabove, the disturbance may be imparted with a piezoelectric transducerwith or without a rod or plunger, or from magnetic, electric or acousticforces. As illustrated, the disturbance grows, resulting in the standingwave on the stream 32 and causing the series of droplets 37, 35 shown.The larger parent droplets 37 are typically on the order of twice thediameter of the orifice 30, whereas the smaller satellite droplets 35have diameters much smaller than the orifice 30.

[0029] Depending on the characteristics of the excitation disturbance, asatellite droplet 35 will merge with the forward or rearward parentdroplet 37 to form a merged droplet 34, or can be forced to maintain itsposition between the forward and rearward parent droplets 37 using anappropriate application of harmonics on the axisymmetric disturbance. Anexample of such a disturbance is one having second and third orderharmonics, although many other disturbances are possible. In the exampleof FIG. 2, the satellite droplets merge with a parent droplet within onewavelength, λ, of the excitation disturbance. The merging time and thediameter of the satellite droplets can be manipulated by the waveformconditions. For example, waveforms composed of added harmonics, orwaveforms with very high driving amplitudes, which effectively distortthe linearity of the disturbance will affect the properties of thestellite droplets. Accordingly, the present invention uses harmonicdisturbances to prevent instantaneous merging so that the satellitedroplets can be deflected out of the main stream to separate thesatellite droplets from the parent droplets. The diameter of thesatellite droplet tends to be a function of the characteristics of theexcitation disturbance, while the diameter of the parent droplet tendsto be a function of the excitation disturbance and the nozzle orifice asshown by the following:

r _(d) =[r _(o) ³(8π)/(3k_(o))−r _(s) ³]^(1/3)   (2.0)

[0030] where r_(o) is the orifice radius and r_(s) is the satellitedroplet radius.

[0031] Once the satellite droplets 35 and parent droplets 37 are formed,they are separated, and then the satellite droplets 35 or satellite andparent droplets 35, 37 are cooled, to solidify, and collected. FIG. 3illustrates one method of separating the satellite droplets from theparent droplets using electrostatic forces. A charge electrode 40 islocated near the orifice 30 where droplets 37, 35 break from thecapillary stream 32. The charge electrode 40 allows for an electrostaticcharge to be selectively applied to any of the droplets 37, 35 on adroplet-by-droplet basis. The charge electrode 40 is coupled to thecontroller 26 by electrical connection 42. Because of the highlypredictable nature of droplet formation from capillary stream break-up,the time at which droplets 37, 35 break from the capillary stream 32 isknown to a precise degree, given the function at which the piezoelectriccrystal 20 is driven and other system parameters. More particularly, theperturbation on the stream's radius grows exponentially in time, t, asr(t)=r_(o)±r_(o)κe^(βt), where κ and β are the amplitude of the initialperturbation and the disturbance growth rate, respectively. The time atwhich droplets break from the capillary stream is the time when r(t)=0,i.e., when t=(1/β)ln(1/κ). See M. Orme, “On the Genesis of DropletStream Microspeed Dispersions,” Physics of Fluids, 3, (12) pp 2936-2947,1991.

[0032] It can be appreciated that an electrostatic charge on the chargeelectrode 40 causes a corresponding electrostatic charge on theconductive capillary stream 32. When a droplet 37, 35 breaks from thestream 32, the droplet 37, 35 is effectively short circuited; therefore,the droplet 37, 35 will maintain that electrostatic charge while inflight. Each droplet 37, 35 can thus be selectively charged, determinedby the controller 26, by charging the charge electrode 40 to apredetermined value at the time that each droplet 37, 35 breaks from thestream 32. The electrostatic charge, Q, per mass, m, of each droplet isgiven theoretically by Schneider's Equation: $\begin{matrix}{{\frac{Q}{m} = \frac{2\quad \pi \quad ɛ_{o}V_{c}}{\rho \quad r_{o}^{2}{\ln \left( {b/r_{o}} \right)}}},} & (3.0)\end{matrix}$

[0033] where ε_(o) is the permitivity of free space, V_(c) is the chargepotential, ρ is the fluid density and b is the electrode radius. See J.M. Schneider, N. R. Lindblad, & C. D. Hendricks, “Stability of anElectrified Liquid Jet,” J. Applied Physics. 38, 6, 2599, 1967. Thegraph of FIG. 4 compares measured and predicted results for the chargeper unit mass of the droplets, using the apparatus and method describedherein. As FIG. 4 shows, Schneider's Equation is useful to predict thecharge of the droplets.

[0034] After being electrostatically charged, the droplets 37, 35 ofmolten metal are directed to pass between a pair of deflection plates44. The bias voltage across the deflection plates 44 is controlled bythe controller 26. When a bias voltage is applied across the deflectionplates 44 by electrical connections 46, it can be appreciated that anelectric field is formed between the plates 44. As charged droplets 37,35 pass between the plates 44, and thus through this electric field, thedroplets 37, 35 are acted upon by an electrostatic force. Theelectrostatic force on a droplet is proportional to the electric fieldand to the charge of the droplet.

[0035] The magnitude of the electrostatic force acting on the droplet37, 35 determines the degree to which the droplet 37, 35 isdeflected—from an axis defined by the capillary stream 32—and thus thepath the droplet 37, 35 travels. The deflection (x_(d)) of a chargeddroplet due to the electrostatic field of a pair of deflection platescan be approximated by Fillmore's Approximation: $\begin{matrix}{{x_{d} = {\frac{QE}{{mv}_{o}^{2}}{l_{dp}\left( {z_{p} - \frac{l_{dp}}{2}} \right)}}},} & (4.0)\end{matrix}$

[0036] where l_(dp) is the length of the deflection plates, Q is thecharge, E is the electric field strength, m is the mass, υ_(o) is thedroplet speed and z_(p) is the vertical distance between the deflectionplate and the target. See G. L. Fillmore, W. L. Buehner, & D. L. West,“Drop Charging and Deflection in an Electrostatic Ink Jet Printer,” IBMJ. Res. Dev. January 1977. A more accurate model that considers theeffects of drag is given by the equations: $\begin{matrix}{{m\frac{v_{x}}{t}} = {{QE} - {D\quad \sin \quad \theta}}} & (5.0) \\{{m\frac{v_{z}}{t}} = {{mg} - {D\quad \cos \quad \theta}}} & (5.1) \\{D = {{C_{d} \cdot \frac{1}{2}}\rho_{a}{v^{2} \cdot A}}} & (5.2) \\{C_{d} = {\frac{24}{Re} + \frac{6}{1 + \sqrt{Re}} + 0.4}} & (5.3)\end{matrix}$

[0037] where D is the aerodynamic drag force, g is the gravitationalconstant, θ is the deflection angle measured from the undeflectedstream, A is the frontal surface area of the sphere, C_(d) is thedimensionless drag coefficient, and Re is the dimensionless Reynoldsnumber. See Q. Liu, C. Huang, and M. Orme, “Mutual Electrostatic ChargeInteractions Between Closely Spaced Charged Solder Droplets.” J. ofAtomization and Sprays, Vol. 10 no. 6, pp 565-585, 2000.

[0038] As FIG. 5 shows, this model (Equations 5.0-5.3), whichincorporates drag, very accurately predicts measured deflection values.Fillmore's Approximation (Equation 4.0) also tends to indicate adroplet's deflection, although it tends to underestimate the actualdeflection somewhat.

[0039] For the embodiment shown in FIG. 3, the satellite droplets 35will have higher charge to mass ratios than the parent droplets 37, sothe electrostatic deflection of the satellite droplets 35 will begreater. Accordingly, a collector 48 is provided to catch at least thesatellite droplets 35, preferably after they have solidified to avoiddefects. In one aspect of a preferred embodiment, the collector has afirst section 50 and a second section 52, wherein the first and secondsections 50, 52 are aligned to catch the satellite and parent droplets35, 37, respectively.

[0040] Another method of separating the satellite droplets from theparent droplets is by acoustic forcing. As shown in FIGS. 6a and 6 b,acoustic forcing is used to exploit the rotation imparted onto thecapillary stream 32 as it exists from the orifice 30. The direction ofrotation is shown by arrow A. Due to conservation of angular momentum,increasing the amplitude of the excitation disturbance (as shown in FIG.6a) causes the satellite droplets 35 to be deflected out of the mainstream and away from the parent droplets 37. When the excitationamplitude is reduced (as shown in FIG. 6a), the effects of the rotationare less pronounced, and the satellite droplets do not separate from themain stream. As with the embodiment shown in FIG. 3, a collector 48 isprovided to catch at least the satellite droplets 35, preferably afterthey have solidified to avoid defects. The collector preferably has afirst section 50 and a second section 52, wherein the first and secondsections 50, 52 are aligned to catch the satellite and parent droplets35, 34, respectively.

[0041] Another method of separating the satellite droplets from theparent droplets uses aerodynamic forces, as shown in FIG. 7. Atransverse aerodynamic force is a applied to the satellite droplets 35and parent droplets 37 by, e.g., fans 54, air jets or the like. Becauseof the mass difference between the satellite and parent droplets, thetransverse aerodynamic force is large enough to propel the satellitedroplets 35 out of the main stream, but it is insufficient tosignificantly affect the larger parent droplets 37. A collector 48 isprovided to catch at least the satellite droplets 35, preferably afterthey have solidified to avoid defects.

[0042] With respect to any of the embodiments described, the parentdroplets can be recycled back into the chamber 14 after they arecollected. To avoid impurities, the recycled metal is preferablyfiltered.

[0043] To produce highly uniform and substantially spherical metal ballswith little or no defects, it is important that the droplets solidify inflight. Cooling the spheres in flight avoids the problem of bondingbetween the molten droplets, either in flight or during theircollection. Further, solidifying the droplets before they are collectedavoids defects of their spherical shape that would result from a molten,or partially molten, droplet hitting a hard surface. Therefore, theballs formed in accordance with the present invention are preferablysolidified before being collected. More effective cooling of thedroplets can be accomplished in various ways. For example, lengtheningthe flight path gives the droplets more time to solidify. To furtherfacilitate the cooling of the droplets in flight, the droplets may beactively cooled by directing them through a chamber filled withcryogenic (or otherwise cooled) inert gas.

[0044] Additionally, to more effectively cool the molten droplets inflight, the droplets may be directed in paths that are different fromthe path of their adjacent downstream droplet. This ensures that the airthrough which a droplet passes has not been heated by the precedingdroplet in the series and that each droplet is expelled from theprotective aerodynamic wake of its neighboring or preceding downstreamdrop, thereby allowing each droplet to cool more effectively. Whendiscussing droplets in a series of droplets, downstream droplets areunderstood to be droplets that are produced earlier in the series,whereas upstream droplets are produced later. A droplet thus followsdownstream droplets and is followed by upstream droplets (the “stream”in this case referring to the downward flow of metal). An adjacentdroplet is a droplet in a series of droplets that is immediatelyupstream or downstream in the series. For these definitional purposes,because the satellite and parent droplets are being separated, adroplet's adjacent downstream droplet in either the satellite or parentstream is the droplet produced two cycles of the excitation frequencyearlier, not the droplet produced immediately before it.

[0045] As described above, the droplets 37, 35 can be selectivelycharged by the charge electrode 40. In this example, the droplets 37, 35are charged with an amplitude varying waveform. The waveform by whichthe charge electrode 40—and thereby the droplets 37, 35—are charged isproduced by, e.g., a waveform generator in the controller 26, and itshould be understood that any waveform that varies the charge on thedroplets 37, 35 could be used (e.g., sawtooth, sinusoid, or the like).The charged droplets 37, 35 are then directed through an electrostaticfield (i.e., between a pair of deflection plates), where the droplets37, 35 are acted upon by an electrostatic force.

[0046] In the case of the embodiment of FIG. 3, the charge applied tothe capillary stream and maintained by the droplets 37, 35 is constant.As a result, the pair of deflection plates 44 functions to separate thesatellite droplets 35 from the parent droplets 37. If the charge appliedto the charge electrode 40—and thereby the droplets 37, 35—is variedover time, the pair of deflection plates 44 functions to vary thedeflection or path of adjacent satellite droplets 35 and parent droplets37 in their respective streams, in addition to separate the satellitedroplets 35 from the parent droplets 37. Alternatively, a second pair ofdeflection plates orthogonally oriented to the first pair of deflectionplates 44 could be used to further disperse the droplets 37, 35 on asecond axis orthogonal to the first. In another alternative embodiment,the deflection plates 44, in the case of a single pair of deflectionplates, may rotate to radially deflect the droplets. In the case of twopairs of deflection plates, preferably the second pair may rotate toradially deflect the droplets.

[0047] Other alternatives may include applying electrostatic charges tothe droplets 37, 35 that are constant while driving the deflectionplates with a varying bias voltage to vary the deflection of thedroplets 37, 35. In the case of two pairs of deflection plates, anotheralternative would be to drive the first pair of deflection plates with aconstant bias voltage to separate the satellite droplets 35 from theparent droplets 37, while the second pair of deflection plates is drivenat a varying bias voltage to vary the deflection of the droplets 37, 35.

[0048] A significant advantage of the present invention is that itenables the production of micro-metallic spheres that are significantlysmaller than the diameter of the orifice from which they emerge, i.e.,ball diameters preferably in a range of about 1.0 to 100 microns andpreferably less than about 25 microns. Because much smaller spheres canbe produced with larger diameter orifices, the difficulties plaguingsmaller orifices, such as orifice clogging, tend to be avoided with thepresent invention. Additionally, an advantage of the present inventionis that the micro-metallic balls, i.e., satellite or satellite andparent combined, can be produced at very high rates several orders ofmagnitude greater than conventional methods, i.e., preferably on theorder of tens of thousands of balls per second, while still maintaininga high degree of uniformity without having to perform an additional stepof mechanically sieving or sorting. More particularly, themicro-metallic balls may be produced in accordance with the presentinvention at a rate preferably in a range of about 1000 to 200,000 ballsper second and preferably at a rate greater that 4000 balls per second,while the ball diameter may be maintained within a tolerance of a meanball diameter preferably in the range of about 0.5 to 3.0 percent andpreferably less than 2.0 percent.

[0049] While the invention is susceptible to various modifications andalternative forms, a specific example thereof has been shown in thedrawings and is herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the appended claims.

What is claimed is:
 1. A ball comprising: a metal, wherein the ball hasa diameter in a range of about 1.0 micron to less than 50 microns. 2.The ball of claim 1 wherein the ball has a diameter of less than 25microns.
 3. The ball of claim 1, wherein the ball is substantiallyspherical.
 4. The ball of claim 1, wherein the ball is formed by aprocess of capillary stream break-up wherein the diameter of the ball isless than two times the diameter of an orifice from which the ball wasformed.
 5. The ball of claim 1, wherein the ball is formed by a processof capillary stream break-up wherein the diameter of the ball is lessthan the diameter of an orifice from which the ball was formed.
 6. Ametallic ball formed from a process of capillary stream break-up whereinthe diameter is in the range of about 1.0 to 50.0 microns.
 7. The ballof claim 6 wherein the diameter of the ball is less than 25 microns. 8.A metallic powder comprising: a plurality of metal balls each having adiameter in the range of about 1.0 to 100 microns, and wherein thediameters of each of the plurality of metal balls is within a range ofabout 0.5 to 3.0 percent of a mean ball diameter.
 9. The metallic powderof claim 8, wherein each ball of the plurality of metal balls issubstantially spherical in shape.
 10. The metallic powder of claim 8,wherein the diameter of each ball of the plurality of metal balls isless than 50 microns.
 11. The metallic powder of claim 8, wherein thediameter of each ball is less than 25 microns.
 12. A method ofmanufacturing ultra-small metallic spheres comprising the steps of:forming parent and satellite droplets from a capillary stream of moltenmetal; separating the satellite droplets from the parent droplets;cooling the satellite droplets to solidify the balls; and collecting thesatellite balls.
 13. The method of claim 12, wherein the forming stepcomprises the steps of directing a capillary stream of molten metal froman orifice by applying an excitation disturbance, wherein the excitationdisturbance is determined so that parent droplets and satellite dropletsform from the stream due to capillary stream break-up;
 14. The method ofclaim 12, wherein the separating step comprises the steps of: impartingan electrostatic charge to the satellite and parent droplets; anddeflecting the satellite and parent droplets by directing the dropletsthrough an electric field.
 15. The method of claim 14, wherein thedeflecting step includes the steps of varying the electric field. 16.The method of claim 14, wherein the deflecting step includes the stepsof applying a constant electric field and varying the electric charge onthe droplets.
 17. The method of claim 14, wherein at least a portion ofthe electric field is created by applying a voltage across a pair ofdeflection plates, the satellite and parent droplets being directedthrough the pair of deflection plates.
 18. The method of claim 12,wherein the separating step comprises increasing the magnitude of theexcitation disturbance to thereby increase rotation of the capillarystream as it exists the orifice, wherein the satellite droplets aredeflected from the parent droplets.
 19. The method of claim 12, whereinthe separating step comprises applying an aerodynamic force to thesatellite and parent droplets, the aerodynamic force having at least acomponent in a direction orthogonal to the capillary stream.
 20. Themethod of claim 12, wherein the excitation disturbance comprisesharmonic disturbances.
 21. The method of claim 11, wherein the collectedsatellite balls have a diameter in a range of about 1 to 100 microns.22. The method of claim 21, wherein the collected satellite balls have adiameter of less than 25 microns.
 23. The method of claim 12, whereineach diameters of the satellite balls is within a range of about 0.5 to3.0% of a mean ball diameter.
 24. The method of claim 12, furthercomprising the step of recycling the parent droplets back into themolten metal.
 25. The method of claim 24, further comprising the step offiltering the molten metal.
 26. The method of claim 12, wherein thecooling step comprises actively cooling at least the satellite dropletsin flight by directing the satellite droplets through a chamber filledwith a cooled gas.
 27. A plurality of metal balls, each havingsubstantially the same diameter in a range of about 1.0 to 100 microns,wherein the plurality of balls is produced by a process comprising thesteps of: directing a capillary stream of molten metal from an orificeby applying an excitation disturbance, wherein the excitationdisturbance is determined so that parent droplets and satellite dropletsform from the stream due to capillary stream break-up; separating thesatellite droplets from the parent droplets; and cooling the satellitedroplets to form solid balls of substantially spherical shape.
 28. Theplurality of metal balls of claim 27, wherein the diameters of the metalballs are within a range of about 0.5 to 3.0% of a mean ball diameter.29. The plurality of metal balls of claim 28, wherein the separatingstep is performed, at least in part, by electrostatic deflection. 30.The plurality of metal balls of claim 27, wherein the separating step isperformed, at least in part, by acoustic forcing.
 31. The plurality ofmetal balls of claim 28, wherein the separating step is performed, atleast in part, with aerodynamic forces.